Highly delaminated poly(dicyclopentadiene)/clay nanocomposites were synthesized by in situ polymerization of dicyclopentadiene/organically modified montmorillonite clay dispersions. Dicyclopentadiene/clay suspensions were sonicated for various times to enhance the degree of delamination prior to curing. The d spacings of the clay in nanocomposites were monitored using X-ray diffraction (XRD), and the extent of delamination was examined by transmission electron microscopy (TEM) and preliminary neutron scattering studies. A new approach, use of confocal laser microscopy, was employed to follow the dispersion of clay layers, tagged by a fluorescent dye, within the liquid monomer. It is evident that XRD cannot be used alone as a criterion for exfoliation. TEM showed that increasing the clay concentration at constant sonication gave an increase in average tactoid size. The largest improvement in composite mechanical properties occurred at clay loading levels (0.5-1 wt %). Significant increases in T-g, elastic bending moduli, flexural moduli, and flexural strengths were found at 0.5-1 wt % clay loadings, where the highest degree of delamination/exfoliation also occurred.